The invention deals with a servovalve with a torque motor with mechanical characteristic adjustment, particularly suitable for high pressure levels. The torque motor consists of a square or rectangle cross-section armature surrounded by four pole shoes, two of which are pole shoes with permanent magnets of high energy density located against each other, and the other two are electric pole shoes with exciting coils, also facing each other, but positioned by 90° to the magnetic ones. All the pole shoes are located on the other side than the armature in an outer ring thread, which thus closes the magnetic field. The existing solutions of torque motors of servovalves have to deal with two basic problems, namely separation of working liquid from the servovalve electromagnetic circuit, and then adjustment of the torque motor electromagnetic circuit.
Separation of the liquid area from the electromagnetic circuit of the servovalve is usually solved by sealing integrated in the torque motor armature attachment. There are two solutions. In the first one the attachment consists of a flexible thin-walled tube, the wall of which separates the work medium space from the electromagnetic part of the torque motor. In the other one the attachment consists of a thin flexible sheet, while a static sealing (an O-ring) is installed in the armature rotation axe, and the armature goes through the sealing. However these solutions are not suitable for higher pressure levels of several MPa.
Another solution with the magnetic circuit inundated in the work medium has to be used for high pressures. Actually it is necessary to use specially impregnated coils and to seal wires going through the case of the product.
Adjustment of torque motors is necessary for achieving the required armature stroke characteristic in relation to the controlling signal, mostly electric current. For manufacturing purposes each product has to be finely adjusted to the required characteristics, which have to be quite precise. The existing solutions usually change the electromagnetic power by changing magnetization of the permanent magnets in the electric circuit. This solution requires use of magnets, which can be magnetized and demagnetized. However these magnets are always bigger and heavier than those made of rare soils, which are magnetized from manufacturing and have high energy density relative to a volume unit. In some instances the torque motor characteristic pattern has to be modified, which can only be achieved by different distance between the armature and the pole shoes, which however requires to dismantle the torque motor, insert inlets or grind off the pole shoes.
The above reasons call for providing a solution of a torque motor that could cope with work medium pressures of up to several MPa with no impact on the operation or life cycle of the servovalve components.
It is then necessary to find a torque motor design enabling adjustment of torque motor characteristics without having to dismantle it, modify its parts or magnetize the permanent magnets.
Finally we should find such a solution of torque motor that uses permanent magnets of rare soils which have high energy density, which can reduce dimensions and weight of the motor.
The primary task of the invention is thus to develop a servovalve with electromagnetic torque motor mentioned in the introductory section, where the new solution separates the work liquid area from the servovalve electromagnetic circuit and facilitates adjustment of the servovalve. The next task of the invention is to minimize the work liquid flow in the area of the electromagnetic circuit. The first task is solved by a solution consisting except others of the above mentioned magnetic pole shoes, an outer ring, a coil and an armature. The servovalve with torque motor consists of the known cylindrical case, in which the armature is situated on one end of an arm, and the orifice closing the nozzle on the other end of the arm, while the arm is free to rotate around a pivot point of a flexible suspension, and there are two magnetic pole shoes and two electric pole shoes with coils positioned by 90° against the first ones, around the armature inside the cylindrical case. The invention principle is based on the fact that the arm with the armature are surrounded by the working liquid, which fills in the inside of the cylindrical case, the pole shoes are fitted with sealing rings on their ends protruding inside the cylindrical case, and the coils are placed outside the case, i.e. outside the area filled in with the working liquid.
In one advantageous modification of the pole shoes attachment the pole shoes are equipped with a threaded part with male thread on their ends oriented from the armature, fitting to the female thread of the outer ring fitted on the cylindrical case.
In another advantageous modification of the pole shoes attachment the pole shoes are equipped with a threaded part with male thread on their ends oriented from the armature, passing through the outer ring and fitting to the female thread cut in the cylindrical case.
In another advantageous modification of the invention the pole shoes are equipped with an axial housing made of non-magnetic material.
The main advantage of the solution of an electromagnetic servovalve with a torque motor according to the invention is, that it enables the servovalve to operate in operation pressures of up to tens of MPa, while the working liquid is separated from the coils and adjustment points of the torque motor.
Another advantage of the solution of an electromagnetic servovalve with a torque motor according to the invention is, that the servovalve characteristics may be simply adjusted by turning the individual pole shoes and thus changing their distance from the armature.
Another advantage of the solution according to the invention is the possibility to use permanent magnets with high energy density, and these magnets do not have to be re-magnetized during adjustment.
The second task of the invention, namely restriction of working liquid flow through the electromagnetic circuit area is solved by the advantageous servovalve solution according to the invention, where the diameter of the hole in the flexible suspension through which the armature arm goes is not bigger than necessary for enabling the required deflection of the armature arm.
The advantage of this servovalve solution is in limitation of the working liquid flow near the armature and the pole shoes to the minimum, which prevents possible impacts on the electromagnetic field and accumulation of magnetic impurity near the armature and the pole shoes.